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From: Finn Thain <fthain@linux-m68k.org>
To: Peter Zijlstra <peterz@infradead.org>, Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Mark Rutland <mark.rutland@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	linux-m68k@vger.kernel.org, linux-doc@vger.kernel.org
Subject: [RFC v3 1/5] documentation: Discourage alignment assumptions
Date: Wed, 08 Oct 2025 09:19:20 +1100	[thread overview]
Message-ID: <76571a0e5ed7716701650ec80b7a0cd1cf07fde6.1759875560.git.fthain@linux-m68k.org> (raw)
In-Reply-To: <cover.1759875560.git.fthain@linux-m68k.org>

Discourage assumptions that simply don't hold for all Linux ABIs.
Exceptions to the natural alignment rule for scalar types include
long long on i386 and sh.
---
 Documentation/core-api/unaligned-memory-access.rst | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/Documentation/core-api/unaligned-memory-access.rst b/Documentation/core-api/unaligned-memory-access.rst
index 5ceeb80eb539..1390ce2b7291 100644
--- a/Documentation/core-api/unaligned-memory-access.rst
+++ b/Documentation/core-api/unaligned-memory-access.rst
@@ -40,9 +40,6 @@ The rule mentioned above forms what we refer to as natural alignment:
 When accessing N bytes of memory, the base memory address must be evenly
 divisible by N, i.e. addr % N == 0.
 
-When writing code, assume the target architecture has natural alignment
-requirements.
-
 In reality, only a few architectures require natural alignment on all sizes
 of memory access. However, we must consider ALL supported architectures;
 writing code that satisfies natural alignment requirements is the easiest way
@@ -103,10 +100,6 @@ Therefore, for standard structure types you can always rely on the compiler
 to pad structures so that accesses to fields are suitably aligned (assuming
 you do not cast the field to a type of different length).
 
-Similarly, you can also rely on the compiler to align variables and function
-parameters to a naturally aligned scheme, based on the size of the type of
-the variable.
-
 At this point, it should be clear that accessing a single byte (u8 or char)
 will never cause an unaligned access, because all memory addresses are evenly
 divisible by one.
-- 
2.49.1


  parent reply	other threads:[~2025-10-07 23:50 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-10-07 22:19 [RFC v3 0/5] Align atomic storage Finn Thain
2025-10-07 22:19 ` [RFC v3 4/5] atomic: Add alignment check to instrumented atomic operations Finn Thain
2025-10-07 22:19 ` [RFC v3 3/5] atomic: Specify alignment for atomic_t and atomic64_t Finn Thain
2025-10-07 22:19 ` [RFC v3 2/5] bpf: Explicitly align bpf_res_spin_lock Finn Thain
2025-10-09  2:10   ` Alexei Starovoitov
2025-10-09  2:56     ` Finn Thain
2025-10-09  7:02     ` Peter Zijlstra
2025-10-09 15:17       ` Alexei Starovoitov
2025-10-09 16:01         ` Arnd Bergmann
2025-10-09 16:12           ` Alexei Starovoitov
2025-10-07 22:19 ` Finn Thain [this message]
2025-10-14 10:23   ` [RFC v3 1/5] documentation: Discourage alignment assumptions David Laight
2025-10-15  7:40     ` Finn Thain
2025-10-15 13:53       ` David Laight
2025-10-16  6:53         ` Finn Thain
2025-10-16 13:30   ` Arnd Bergmann
2025-10-16 22:14     ` Finn Thain
2025-10-07 22:19 ` [RFC v3 5/5] atomic: Add option for weaker alignment check Finn Thain

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